In the past a large amount of work has been devoted to the manufacture of sulphur-containing fertilizers. The growing worldwide demand for sulphur-containing fertilizers stems from the discovery that low crop yields in certain cases may be related to deficiencies in sulphur in the soil. An example of a species with high sulphur requirements is Canola. Canola is an important cash crop in Alberta, Canada, and has high sulphur requirements at any growth stage. A shortage of sulphur can cause serious reductions in crop yield and quality.
Manufacturing processes for sulphur-containing fertilizers of the ammonium phosphate type often involve the use or incorporation of sulphates, see e.g. U.S. Pat. No. 4,377,406, or U.S. Pat. No. 4,762,546. A disadvantage of sulphates is that they are very mobile in the soil and easily leach out of the root zone, effectively making the sulphate nutrient unavailable to the plants.
Elemental sulphur is not leached out of the soil, as sulphates are. Instead, micron sized elemental sulphur particles are oxidized to sulphate sulphur, which is the form utilized by the plants, by soil bacteria during the cropping season. Elemental sulphur is, thus, considered a slow release form of plant nutrient sulphur that is less prone to leaching out of the crops root zone. It is, therefore, advantageous to have a large proportion of the sulphur in fertilizers present as elemental sulphur. Furthermore, elemental sulphur offers some additional benefits in agriculture, including acting as a fungicide against certain micro organisms, acting as a pesticide against certain soil and plant pests, assisting the decomposition of plant residues and improving phosphorus and nitrogen nutrient utilization and reducing the pH of alkaline and calcareous soils.
Thus, it is advantageous to incorporate sulphur in sulphur-containing fertilizers as elemental sulphur present as small particles.
Processes for the manufacture of sulphur-containing fertilizers, wherein elemental sulphur is used, are known in the art. Most of the methods involve the incorporation of molten sulphur into the fertilizer.
In U.S. Pat. No. 5,653,782, a process for the manufacture of sulphur-containing fertilizers has been described, wherein a substrate containing fertilizer particles is heated to a temperature above the melting point of sulphur and admixed with sulphur. According to U.S. Pat. No. 5,653,782, the sulphur is melted by the heat provided by the preheated fertilizer particles, thereby producing a homogeneous coating on the fertilizer particles.
U.S. Pat. No. 3,333,939, describes the coating of ammonium phosphate granules with molten sulphur. The granules are coated in a separate coating unit into which the sulphur is fed, by contacting the granules with molten sulphur or with a solution of ammonium polysulphide. Subsequently, the coated granules are dried.
Alternatively, U.S. Pat. No. 3,333,939 teaches a process for preparing sulphur-containing fertilizer particles in which the sulphur is interspersed throughout the particles. In this process ammonia and phosphoric acid are allowed to react to form ammonium phosphate. The ammonium phosphate formed is fed into a granulator in which it is mixed with urea and dry sulphur. The granules obtained are dried in a dryer.
The disadvantage of the first process of U.S. Pat. No. 3,333,939 is that the coating prevents a uniform distribution of ammonium sulphate and sulphur into the soil. The second process has the disadvantage that it requires solid sulphur handling. The handling and grinding of solid elemental sulphur is highly hazardous due to the generation of sulphur dust and risks of sulphur dust fires and explosions. As mentioned in a review by H. P. Rothbaum et al (New Zealand Journal of Science, 1980, vol. 23, 377), explosion hazards are always due to sulphur dust which is inflammable. Therefore, a complex process design is necessary to ensure the safety of the process.
U.S. Pat. No. 5,571,303 discloses a process for the manufacture of fertilizers in which first ammonia, water and phosphoric acid are reacted to form ammonium phosphate. Subsequently, the ammonium phosphate/water mixture is mixed with molten sulphur. The mixture thus obtained is kept at temperatures of 120-150° C. until granulation. A disadvantage of this process is safety, that is elevated sulphur concentrations in process dusts may lead to potentially explosive dust-air mixtures.
EP 1560801 A1 discloses a process for the manufacture of sulphur-containing ammonium phosphate fertilizers comprising combining elemental sulphur, in the liquid form, with ammonia, phosphoric acid and water.
Several prior art documents, e.g. GB 1312314, US 2002/0139158, WO 97/16396, WO 02/090295, U.S. Pat. No. 5,423,897 and U.S. Pat. No. 3,926,841 have disclosed the use of coating agents, incorporating surfactants, in the reduction of dust formation and caking during use and handling of NPK-type fertilizers.
WO 2008/024007 discloses a sulphur fertilizer, either in the form of globular granules or pellets that constitutes solidified mixtures of liquid sulphur and bentonite, and favourable additional components and fertilizing microelements, that is characterised by its content of 60 to 95% of sulphur, 4 to 20% bentonite and 1 to 8% lignosulphonate. The lignosulphonate is incorporated into the sulphur fertilizer molecules in order to allow the formation of granules or pellets which disperse easily in contact with soil moisture into sulphur molecules of much smaller size than known fertilizers of this type.
GB 2292140 also discloses sulphur/bentonite fertilizer compositions in which 1 to 30 wt. % of a binding material, such as a lignosulphonate, may be added in order to form suitable granules for application of sulphur to the land.
US 2004/0009878 describes particles containing a nitrogen-containing fertilizer composition, preferably based on urea, in which a binder component, such as a lignosulphonate, is present such that contact of the particles with water causes particle dispersion into more than 100 pieces.
WO 90/03350 discloses a sulphur-based chemical soil-corrective in the form of pellets for agricultural use, said product comprising fly sulphur powder, at least 3 wt. % of an inert product selected from the group consisting of clay, bentonite, kaolin and mixtures thereof, and at least 0.5 wt. % of a wetting agent, said components being mixed with one another and subjected to wet extrusion and subsequent drying to obtain said pellets. The wetting agent may be a lignin sulfonate. However, as WO 90/03350 relates to the handling of sulphur powder which is subjected to extrusion (rather than granulation as used according to the present invention) the teachings therein are not relevant for the process according to the invention.
Regardless of the improvements disclosed in the art, problems with the manufacture of phosphate-based fertilizers containing elemental sulphur, continue to be in existence. In particular, dust and explosion hazards involving elemental sulphur dust continue to be of great concern. Therefore, there remains a need for fertilizers and manufacturing processes for such fertilizers which would diminish or even prevent the safety problems experienced in the art.